Unnamed dwarf planet may be third largest in the solar system

What’s a planet gotta do to get a moniker? New observations of a body beyond the orbit of Neptune, known only as 2007 OR10, suggest it is could be the third largest dwarf planet in the solar system, following Pluto and Eris – and yet it still doesn’t have a proper name.

The diminutive world is one of a gaggle of trans-Neptunian objects (TNOs) discovered by a team led by Mike Brown at the California Institute of Technology in the 2000s. Its extreme distance – until recently it was considered the second most distant object in the solar system – made it hard to measure its size.

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Brown’s team originally pegged 2007 OR10’s diameter at around 1200 kilometres, making it smaller than four of the five officially designated dwarf planets: Pluto, Eris, Haumea and Makemake. The asteroid Ceres, the fifth, is only around 950 km wide.

But now a team led by András Pál of the Konkoly Observatory in Budapest, Hungary have used data from the Kepler and Herschel space telescopes to bump 2007 OR10 up to third place, with a diameter of 1535 km. Kepler is normally tasked with spotting planets outside our cosmic neighbourhood, but was recently repurposed to occasionally look across the plane of our solar system, giving it an eye on 2007 OR10.

The team used this data to measure 2007 OR10’s rotation rate for the first time, discovering it has a relatively slow day of 44.8 hours. That long day, combined with thermal data from Herschel, increases the dwarf’s likely size, says Pál. “If the rotation is slower it means the heat is redistributed in different ways.”

What’s in a name?

“I’m a little sceptical of the size measurement,” says Brown, as Pál’s readings show it could be between 1310 and 1610 km. “I think with the uncertainty it could easily be smaller than Makemake.”

As discoverers, Brown’s team have first dibs at naming the object. Originally they gave 2007 OR10 the unofficial nickname “Snow White” as it was thought it must be particularly bright to show up in the team’s survey, and was the “seventh dwarf” discovered. But later observations revealed it is actually one of the reddest objects in the solar system, likely due to methane ice on the surface.

As such, we need to find out just what makes 2007 OR10 special before we can give it a name, says Brown. “All of the other objects out there we’ve named have had some sort of appropriate name based on some feature,” he says. “But we will learn more soon, I have hope.”

Pál agrees the error bars on their measurement may end up making 2007 OR10 smaller, but it still deservers a name, even if it means getting a bit creative. “If you consider that there are more and more objects known by astronomers, it’s much harder to find some property in which it is dominant.” 2007 OR10 is one the darkest objects of its size, which combined with its redness could provide a few pointers, he says.